Disclosed are a virtual computer system and method, wherein computer resources are automatically and optimally allocated to logical partitions according to loads to be accomplished by operating systems in the logical partitions and setting information based on a knowledge of workloads that run on the operating systems. Load measuring modules are installed on the operating systems in order to measure the loads to be accomplished by the operating systems. A manager designates the knowledge concerning the workloads on the operating systems through a user interface. An adaptive control module determines the alalocation ratios of the computer resources relative to the logical partitions according to the loads and the settings, and issues an allocation varying instruction to a hypervisor so as to thus instruct variation of allocations.